376 CHEMICAL ENGINEERING
steam pressure of 5 Ib. and a vacuum of from 26 to 27 in. Salt crystals are recovered from the 40 per cent liquor by cooling, and in some cases they have been separated directly in a vertical-tube evaporator and recovered in a standard salt filter. In this case the capacity is from 1 to IK gal. per square foot. In some cases, evaporators have been made of cast iron with steel tubes, but other manufacturers are using evaporators lined with lead or acid-proof brick, and extra-heavy copper and bronze tubes.
Ammonium Nitrate.—The weak liquors are usually concentrated from 25 to 80 per cent in single effects of the horizontal-tube, or special vertical-tube evaporators. The steam pressure is from 5 to 20 Ib. and the vacuum from 18 to 26 in. Evaporators have been built with cast-iron shells and cast-iron tubes, and also with aluminum tubes and enameled bodies.
Ammonium Sulphate.—Gas-producer plants will furnish weak liquors with about 30 per cent sulphate, and these liquors are concentrated in a vertical-tube evaporator%ntil the salt crystals separate and drop into the salt filter attached to the evaporator. With a steam pressure of 5 Ib. and a vacuum of 27 to 28 in., the capacity is from 1 to IK gal. per square foot. Evaporator shells are made of cast iron with tubes of bronze. Frequently the whole apparatus is lead-lined and the tubes are of lead, in which case the capacity will be reduced to about 60 per cent of the above figures.
Aluminum Sulphate.—The liquor usually contains free sulphuric acid and is concentrated from 10 to 45 per cent. It will solidify at 48 per cent. The evaporators must be made of lead entirely and the capacity will be K gal. per square foot with a steam pressure of 5 Ib. and a vacuum of 26 in. Vertical-tube and horizontal-tube machines may be used.
Barium Chloride.—The usual concentration is from 10 to 50 per cent, and with a horizontal-tube evaporator the capacity will be 1 gal. per square foot, with a steam pressure of 5 Ib. and a vacuum of 26 in. Number of effects depends on quantity of liquor. Evaporators are built of cast iron or steel, with steel or wrought-iron tubes.
Calcium Chloride.—The weak liquors will contain from 7 to 10 per cent of solids, and are concentrated to from 43 to 48 per cent. Sodium chloride is separated and recovered in standard salt filters. For the precoricentration, a vertical-tube and rapid-circulation type should be used; for the final concentration, a vertical-tube machine only. The capacity is 2 gal. per square foot for the weak liquor, and 1 gal. for the final concentration, assuming a steam pressure of 10 Ib. and a vacuum of 28 in. The boiling point of calcium chloride increases considerably with the concentration, and the liquors usually separate a heavy scale of calcium sulphate, which has to be removed by mechanical cleaning.
Calcium and Magnesium Chloride (Mother liquors from salt works).— Weak liquor 29°B6., heavy liquor 450Be\ The concentration is done in horizontal-tube and vertical-tube single effects at a capacity of 1 gal. per square foot, with a steam pressure of 10 Ib. and a vacuum of 26 in. Sodium chloride will separate. Final concentration to 59°Be. is done in open pans with high-pressure steam. All parts must be of cast iron or wrought iron, as copper will discolor the product.
Magnesium Sulphate (Epsom salt).—Concentrated from 48 per cent or less to 75 per cent in vertical-tube or rapid-circulation type evaporators. With a